For keeping the environment of the earth, use of solar energy is getting popular. Solar cells are implemented on more and more roofs or walls of ordinary buildings and houses. Among the solar cells, development and production of solar cells which incorporate semiconductors that are advantageous to enlarging of size are rapidly progressing as a leading candidate for producing clean energy.
However, in production process of a solar cell, it is difficult to avoid occurrence of defects in the solar cell. This prevents efficient production of solar cells having high performance and high reliability. Further, if defects occur in a solar cell that has been installed during use of the solar cell, performance and reliability of the solar cell decline.
Conventionally, so-called EBIC (Electron Beam Induced Current) and LBIC (Laser Beam Induced Current), that is, methods for measuring a current or voltage induced by using an electron beam or laser beam and thereby analyzing diffusion length of minority carriers and defects (grain boundary/transgranular), are widely used as the method for detecting/evaluating the defects of the solar cell, for example. By the EBIC or LBIC, it is possible to measure a degree of an elective activity or diffusion length of the minority carriers in solar cells locally and to evaluate conversion efficiency and quality of the solar cell (see Non Patent Literature 1). Moreover, an apparatus has been revealed, which apparatus analyzes, based on infrared light intensity, distribution of heat generated due to a bias in a forward direction, so as to detect a short circuit section (see Non Patent Literature 2). Furthermore, a technique has also been revealed that a back side of a substrate is exposed to strong light so as to detect leakage of light, which as a result detects a substrate crack (see Non Patent Literature 3).
The inventors of the present invention found that, in a case where an electric current is passed through a solar cell in a forward direction, luminescence is observed even under normal carrier introducing condition at room temperature, and developed a technique for examining a defect of a solar cell by analyzing an image of the luminescence (see Patent Literature 1).
By the way, defects in solar cells are generally largely classified into defects caused by internal factors (internal cause defects) and defects caused by external factors (external cause defects). The internal cause defects are defects such as crystal defect, dislocation, grain boundaries and the like that are caused by physical properties of a solar cell. The internal cause defects affect functions of a solar cell, but do not affect reliability of materials constituting the solar cell very much. Meanwhile, the external cause defects are mechanical defects of solar cells such as a substrate crack (e.g., microcrack), electrode rupture, loose connection and the like. The external cause defects cause adverse effect on reliability of solar cells and production yield of solar cells, and therefore become deciding factors in efficient mass production of highly reliable solar cells.
As described above, internal cause defects cause a decline in performance of a solar cell, and causes a deterioration in power generation efficiency of the solar cell if left untreated, but an influence of the internal cause defects is small in terms of long-term reliability. Meanwhile, if external cause defects are left untreated, reliability gradually declines, and in the worst case, a solar cell is broken. Accordingly, external cause defects cause a greater adverse effect than internal cause defects. In order to properly cope with a defect that has occurred in a solar cell during production or use of the solar cell, it is therefore important to clarify whether the defect is an internal cause defect or an external cause defect. However, in the conventional arts, it is impossible to distinguish between an internal cause defect and an external cause defect.
In view of this, the inventors of the present invention improved the technique of Patent Literature 1 and developed a technique in which a direct current is passed through a solar cell in a forward direction and the solar cell is heated, and an internal cause defect and an external cause defect are distinguished from each other by using, as an indicator, light emission characteristics based on a change of a heating temperature (Patent Literature 2). According to this technique, an internal cause defect, which depends on a change of temperature, becomes unclear as the temperature rises, whereas an external cause defect, which does not depend on a change of temperature, appears prominently in a high temperature region. It is thus possible to distinguish between an internal cause defect and an external cause defect.